The invention relates to hydraulic lift mechanism and in particular to such mechanism as is required to serve intermittent alternating displacement of a load, wherein the load may be of various magnitudes within the capacity of the mechanism. Such conditions exist for hydraulically operated cranes and hoists, and the present invention is particularly concerned with bridge cranes, namely, crane constructions in which two transversely spaced parallel longitudinal rails are spanned by a travelling horizontal bridge with end-support means riding these rails, and with a hoist trolley and hook movably supported for guided movement along the bridge.
Conventionally, the bridge structure of high-capacity cranes of the character indicated comprises two spaced parallel bridge girders, joined at their respective ends by the end-support means which rides the spaced rails. Each of these girders incorporates one of two similarly spaced rails for transverse travelling displacement of a hoist carriage, and independent high-load and low-load cable-winch systems are mounted at spaced locations on the carriage. For example, a bridge crane of 100-ton capacity will typically have a first winch system and cable with hook for high loads (100-ton limit) and a second winch system and cable with hook for low loads (25-ton limit). The reason for the dual provision of high and low capacity systems is not only for economy, but also because the high-load system is necessarily relatively slow, as compared with the lifting speed of the low-capacity system. Typically, such a 100-ton crane will commit in the order of 100 tons to the weight of the carriage alone, and for a bridge of 60-foot span, the bridge-girder commitment will be in the order of 40 tons. Most of the energy required to operate the crane is thus committed to moving the crane and its hook, and the particular load represents at most less than half the energy requirement to operate the crane without a load. Typically, for a high duty-cycle crane, the 100-ton winch system will operate at 18 feet per minute, and the 25-ton winch system will operate at 30 feet per minute. Furthermore, inertia throughout each of the lift systems, being electric-motor driven through substantial reduction gearing, compels major time consumption for the acceleration phase and for the deceleration phase of any given lifting or descending operation, regardless of whether or not loaded at the involved hook.